Non-invasive photoelectric pulse oximetry has been previously described in U.S. Pat. No. 4,407,290, U.S. Pat. No. 4,266,554, U.S. Pat. No. 4,086,915, U.S. Pat. No. 3,998,550, U.S. Pat. No. 3,704,706, European patent application Ser. No. 102,816 published Mar. 13, 1984, European patent application No. 104,772 published Apr. 4, 1984, and European patent application No. 104,771 published Apr. 4, 1984. Pulse oximeters are commercially available from Nellcor Incorporated, Hayward, Calif., and are known as, for example, Pulse Oximeter Model N-100.
Pulse oximeters typically measure and display various blood flow characteristics including but not limited to blood oxygen saturation of hemoglobin in arterial blood, volume of individual blood pulsations supplying the flesh, and the rate of blood pulsations corresponding to each heartbeat of the patient. The oximeters pass light through human or animal body tissue where blood perfuses the tissue such as a finger, an ear, the nasal septum or the scalp, and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that is absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For example, the Nellcor N-100 Pulse Oximeter measures oxygen saturation of hemoglobin using two light emitting diodes ("LED's"), one having a discrete frequency of about 660 nanometers in the red light range and the other having a discrete frequency of about 925 nanometers in the infrared range. The two LED's are illuminated alternately with a four-state clock so that the incident light will pass through a fingertip and the detected or transmitted light will be detected by a single photodetector. The clock uses a high strobing rate, e.g., two thousand cycles per second, to be easily distinguished from other light sources. The photodetector current changes in response to both red and infrared transmitted light, in sequence, and is then amplified and separated by a two-channel synchronous detector--one channel for processing the red light waveform and the other channel for processing the infrared light waveform. The separated signals are filtered to remove the strobing frequency, electrical noise, and ambient noise and then digitized by an analog to digital converter ("ADC"). As used herein, incident light or transmitted light refers to light generated by the LED or other light source, as distinguished from ambient or environmental light.
The light source intensity may be adjusted to accommodate variations among patients' skin color, flesh thickness, hair, blood, and other variants. The light transmitted is thus modulated by the variants, particularly the arterial blood pulse or pulsatile component, and is referred to as the optical signal. The digital representation of the optical signal is referred to as the digital optical signal. The portion of the digital optical signal that refers to the pulsatile component is labeled the optical pulse.
The digital optical signal is processed by the microprocessor of the Nellcor N-100 Pulse Oximeter in order to identify individual optical pulses and to compute the oxygen saturation from the ratio of maximum and minimum pulse levels as seen by the red wavelength compared to the pulse seen by the infrared wavelength.
Several alternate methods of processing and interpreting optical signal data have been disclosed in the patents and references cited above.
A problem with non-invasive pulse oximeters is that the optically derived pulse rate may be subject to irregular variants that interfere with the detection of the blood flow characteristics including but not limited to motion artifact. Motion artifact is caused by the patient's muscle movement proximate to the oximeter sensor, for example, the patient's finger, ear or other body part to which the oximeter sensor is attached, and may cause spurious pulses that are similar to pulses caused by arterial blood flow. These spurious pulses, in turn, may cause the oximeter to process the artifact waveform and provide erroneous data. This problem is particularly significant with infants, fetuses, or patients that do not remain still during monitoring.
A second problem exists in circumstances where the patient is in poor condition and the pulse strength is very weak. In continuously processing the optical data, it can be difficult to separate the true pulsatile component from artifact pulses and noise because of a low signal to noise ratio. Inability to reliably detect the pulsatile component in the optical signal may result in a lack of the information needed to calculate blood constituents.
It is well known that electrical heart activity occurs simultaneously with the heartbeat and can be monitored externally and characterized by the electrocardiogram ("EKG") waveform. The EKG waveform, as is known to one skilled in the art, comprises a complex waveform having several components that correspond to electrical heart activity. The QRS component relates to ventricular heart contraction. The R wave portion of the QRS component is typically the steepest wave therein, having the largest amplitude and slope, and may be used for indicating the onset of cardiovascular activity. The arterial blood pulse flows mechanically and its appearance in any part of the body typically follows the R wave of the electrical heart activity by a determinable period of time. See, e.g., Goodlin et al., "Systolic Time Intervals in the Fetus and Neonate", Obstetrics and Gynecology, Vol. 39, No. 2, February 1972, where it is shown that the scalp pulse of fetuses lag behind the EKG "R" wave by 0.03-0.04 second, and U.S. Pat. No. 3,734,086.
It is therefore an object of this invention to provide an improved method and apparatus for detecting the pulsatile component of the optical signal and measuring the amount of blood constituent and the pulse rate by incorporating the patient's heart activity, preferably detected electrically in the form of an EKG waveform, into the oximeter operation and thereby solve problems caused by motion artifact and low signal to noise ratio, as well as simplify and improve the operation of oximeters.
Another object of this invention is to have the oximeter analyze only those digital optical signals occurring during a period of time when the optical pulses are expected to be found and use information from that portion of the signal to calculate the amount of blood constituent. This increases the likelihood that the oximeter will process only optical waveforms that contain the pulsatile component of arterial blood, and will not process spurious pulses.
Another object of the invention is to provide for using pulse oximeters to monitor patients having irregular heartbeats by using the EKG information, particularly the R wave component, to determine when an arterial pulse is likely to occur and processing the digital optical signal waveform during that time period to make the desired measurement.
A further object of this invention is to cross correlate the pulse rate information determined by the oximeter from the digital optical signal with the heart rate determined from the EKG. The cross correlation function will allow measurement of the time relationship between the EKG and the optical pulse and is particularly advantageous when the optical signal may be weak and in the delivery room where fetal heart rate is an important and commonly monitored vital sign.
A further object of this invention is to provide for redundant measurement of the heart rate from both the optical signal and the EKG to continuously monitor the patient even if one of the signals were to be lost.
A further object of this invention is to provide a polarity compensation circuit for use with EKG detection so that the polarity of the EKG waveform can be made uniform, upgoing or downgoing, without having to adjust the leads.